Not Applicable.
The present invention relates to improved methods and apparatus for completing wells in unconsolidated subterranean zones. More particularly, the present invention relates to improved methods and apparatus for achieving effective frac treatments and uniform gravel packs in completing such wells. Still more particularly, the present invention relates to improved methods for achieving effective frac treatments and uniform gravel packs over long and/or deviated production intervals and maximizing the internal production area of the screen assembly by removing an inner flow-control service assembly after treatment.
Oil and gas wells are often completed in unconsolidated formations containing loose and incompetent fines and sand that migrate with fluids produced by the wells. The presence of formation fines and sand in the produced fluids is disadvantageous and undesirable in that the particles abrade and damage pumping and other producing equipment and reduce the fluid production capabilities of the producing zones in the wells.
Completing unconsolidated subterranean zones typically comprises a frac treatment and a gravel pack. A frac/gravel pack apparatus, which includes a sand screen assembly and the like, is commonly installed in the wellbore penetrating the unconsolidated zone. During frac treatment, the zone is stimulated by creating fractures in the rock and depositing particulate material, typically graded sand or man-made proppant material, in the fractures to maintain them in open positions. Then the gravel pack operation commences to fill the annular area between the screen assembly and the wellbore with specially sized particulate material, typically graded sand or man-made proppant. The particulate material creates a barrier around the screen and serves as a filter to help assure formation fines and sand do not migrate with produced fluids into the wellbore. Preferably, to simplify operations, the frac treatment particulate material is the same as the gravel packing particulate material. However, as described herein, the term xe2x80x9cproppantxe2x80x9d refers to the frac treatment particulate material and the term xe2x80x9cgravelxe2x80x9d refers to the gravel packing particulate material.
In a typical frac/gravel pack completion, a screen assembly is placed in the wellbore and positioned within the unconsolidated subterranean zone to be completed. As shown in FIG. 1, a screen assembly 130 and a wash pipe 140 are typically connected to a tool 100 that includes a production packer 120 and a cross-over 110. The tool 100 is in turn connected to a work or production string 190 extending from the surface, which lowers tool 100 into the wellbore until screen assembly 130 is properly positioned adjacent the unconsolidated subterranean zone to be completed.
To begin the completion, the interval adjacent the zone is first isolated. The bottom of the well 195 typically isolates the lower end of the interval or alternatively a packer can seal the lower end of the interval if the zone is higher up in the well. The production packer 120 typically seals the upper end of the interval or alternatively the wellhead may isolate the upper end of the interval if the zone is located adjacent the top of the well. The cross-over 110 is located at the top of the screen assembly 130, and during frac treatment a frac fluid, such as viscous gel, for example, is first pumped down the production string 190, into tool 100 and through the cross-over 110 along path 160. The frac fluid passes through cross-over ports 115 below the production packer 120, flowing from the flowbore of production string 190 and into the annular area or annulus 135 between the screen assembly 130 and the casing 180.
Initially the assembly is in the xe2x80x9csqueezexe2x80x9d position where no fluids return to the surface. In the squeeze position, valve 113 at the top of the wash pipe is closed so fluids cannot flow through wash pipe 140. During squeeze, the frac fluid, typically viscous gel mixed with proppant, is forced through perforations 150 extending through the casing 180 and into the formation. The frac fluid tends to fracture or part the rock to form open void spaces in the formation. As more rock is fractured, the void space surface area increases in the formation. The larger the void space surface area, the more the carrier liquid in the frac fluid leaks off into the formation until an equilibrium is reached where the amount of fluid introduced into the formation approximates the amount of fluid leaking off into the rock, whereby the fracture stops propagating. If equilibrium is not reached, fracture propagation can also be stopped as proppant reaches the tip of the fracture. This is commonly referred to as a tip screen out design. Next a slurry of proppant material is pumped into the annulus 135 and injected into the formation through perforations 150 to maintain the voids in an open position for production.
In a frac treatment, the goal is to fracture the entire interval uniformly from top to bottom. However, because cross-over 110 introduces frac fluid at the top of the formation interval through ports 115 at a very high flow rate, friction causes a large pressure drop as the frac fluid flows down annulus 135 to reach the bottom 195 of the interval. Therefore, more pressure is exerted on the upper extent of the formation interval than on the lower extent of the interval so that potentially full fracturing occurs adjacent the top of the production zone while reduced or no fracturing occurs adjacent the bottom. Additionally, formation strength tends to increase at greater depths such that the longer the zone or interval, the greater the strength gradient between the rock at the top and bottom. Because higher fluid pressures are exerted on the weaker rock at the top, and lower fluid pressures are exerted on the stronger rock at the bottom, the strength gradient adds to the concern that only the upper extent of the interval is being fully fractured. To resolve these problems and achieve more uniform fracturing, it would be advantageous to have a frac apparatus capable of injecting frac fluid into the formation at fairly uniform pressures along the entire interval length from top to bottom. It would also be advantageous to have a frac apparatus capable of continuing to apply frac pressure to the lower extent of the formation even when fractures in the upper interval reach a xe2x80x9ctip screen outxe2x80x9d condition and therefore stop accepting frac fluids or do so at a reduced rate.
Once the frac treatment is complete, the gravel pack commences, or the gravel pack may take place simultaneously with the frac treatment. During gravel pack, the objective is to uniformly fill outer annulus 135 with gravel along the entire interval. Prior to introducing the gravel pack slurry, the assembly is placed in the xe2x80x9ccirculationxe2x80x9d position by opening valve 113 to allow flow through wash pipe 140 back to the surface. The slurry is then introduced into the formation to gravel pack the wellbore. As slurry moves along path 160, out cross-over paths 115 and into annulus 135, the fluid in the slurry leaks off along path 170 through perforations 150 into the subterranean zone and/or through the screen 130 that is sized to prevent the gravel in the slurry from flowing therethrough. The fluids flowing back through the screen 130, enter the inner annular area or annulus 145 formed between the screen 130 and the inner wash pipe 140, and flow through the lower end of wash pipe 140 up path 185. The return fluids flow out through cross-over port 112 into annulus 105 above the production packer 120 formed between the work string 190 and the casing 180, then back to the surface.
The gravel in the slurry is very uniform in size and has a very high permeability. As the fluid leaks off through the screen 130, the gravel drops out of the slurry and builds up from the formation fractures back toward the wellbore, filling perforations 150 and outer annulus 135 around the screen 130 to form a gravel pack. The size of the gravel in the gravel pack is selected to prevent formation fines and sand from flowing into the wellbore with the produced fluids.
During a gravel-packing operation, the objective is to uniformly pack the gravel along the entire length of the screen assembly 130. Conventional gravel packing using cross-over 110 begins at the bottom 195 of the interval and packs upward. However, with a high leak off of fluid through the perforations 150 and into the formation, the gravel tends to deposit around the perforations 150 thus forming a node. A node is a build up of gravel that grows radially and may grow so large that it forms a bridge and completely blocks the outer annulus 135 between the screen 130 and casing 180. Although the primary flow of the gravel pack slurry begins along the axis of the casing 180, to the extent that the flow becomes radial, gravel nodes will build up and grow radially in the outer annulus 135. When the gravel is packed grain to grain to completely block the outer annulus 135 with gravel, that is commonly termed xe2x80x9cscreen outxe2x80x9d in the industry. Bridging or screen out can occur during gravel packing or during frac treatment when the proppant is injected to maintain the voids in an open position. If formation permeability variations and/or the fracture geometry cause a bridge to form in the annulus around the screen during packing, the gravel slurry will begin packing upward from the bridge. This problem occurs particularly in gravel packs in long and/or deviated unconsolidated producing intervals. The resulting incomplete annular pack has sections of screen that remain uncovered, which can lead to formation sand production, screen erosion and eventual failure of the completion.
FIG. 2 illustrates the problem of the formation of gravel bridges 200 in the outer annulus 135 around the screen 130 resulting in non-uniform gravel packing of annulus 135 between the screen 130 and casing 180. This may occur with conventional frac treatments because fractures in the formation do not grow uniformly, and carrier fluid leaks off into high permeability portions of the subterranean zone 210 thereby causing gravel to fill perforations 250 and form bridges 200 in the annulus 135 before all the gravel has been placed along screen 130. The bridges 200 block further flow of the slurry through the outer annulus 135 leaving voids 220, 230 in annulus 135. When the well is placed on production, the flow of produced fluids may be concentrated through the voids 220, 230 in the gravel pack, soon causing the screen 130 to be eroded by pressurized produced fluids and the migration of formation fines and sand into the production string, thus inhibiting production.
In attempts to prevent voids along the screen 130 in gravel pack completions, special screens having external shunt tubes have been developed and used. See, for example, U.S. Pat. No. 4,945,991. The shunt tubes run externally along the outside of the screen assembly and have holes approximately every 6 feet to inject gravel into the annulus between the screen assembly and the wellbore or casing at each hole location. During a gravel pack completion, if the major flow path is blocked because a bridge develops, a secondary or alternative flow path is available through the shunt tubes. If there are voids along the screen below the bridge, gravel can be injected into the annulus through the shunt tube holes to fill the voids to the top of the interval. The holes are sized to restrict the flow out into the annulus and reduce the rate at which fluid leaks off to bridged portions of the overall interval. When screen out occurs at one hole, the shunt tube itself provides an open flow path for the slurry to proceed to the next hole and begin filling the void in that area. When the gravel is packed above the top perforation in the interval, the pressure goes up dramatically, indicating to the operator that the interval is fully gravel packed.
While shunt-tube screen assemblies have achieved varying degrees of success in achieving uniform gravel packs, they are very costly and remain in the well after gravel packing to become part of the permanent assembly. Because shunt tubes are disposed between the screen assembly and the wellbore wall, the internal diameter of the screen assembly is reduced to accommodate the shunt tubes, thereby limiting the available production area, which is especially undesirable in higher production rate wells. It would be advantageous to have a gravel pack apparatus with alternative flow paths that did not reduce or limit the production area of the screen assembly.
Further improved apparatus and methods of achieving uniform gravel packing are shown in U.S. patent application Ser. No. 09/399,674 filed on Sep. 21, 1999, which is a continuation-in-part of Ser. No. 09/361,714 filed on Jul. 27, 1999, which is a continuation-in-part of application Ser. No. 09/084,906 filed on May 26, 1998, now U.S. Pat. No. 5,934,376, which is a continuation-in-part of application Ser. No. 08/951,936 filed on Oct. 16, 1997, now U.S. Pat. No. 6,003,600, all hereby incorporated herein by reference. See also European patent application EP 0 909 874 A2 published Apr. 21, 1999 and European patent application EP 0 909 875 A2 published Apr. 21, 1999, both hereby incorporated herein by reference.
A slotted liner, having an internal screen disposed therein, is placed within an unconsolidated subterranean zone whereby an inner annulus is formed between the screen and the slotted liner. The inner annulus is isolated from the outer annulus between the slotted liner and the wellbore wall and provides an alternative flow path for the gravel pack slurry. The gravel pack slurry flows through the inner annulus and outer annulus, between either or both the sand screen and the slotted liner and the liner and the wellbore wall by way of the slotted liner. Particulate material is thereby uniformly packed into the annuli between the screen and the slotted liner and between the slotted liner and the zone. If a bridge forms in the outer annulus, then the alternative flow path through the inner annulus allows the void to be filled beneath the bridge in the outer annulus.
The permeable pack of particulate material formed prevents the migration of formation fines and sand into the wellbore with the fluids produced from the unconsolidated zone. To prevent bridges from forming in the inner annulus, dividers may be provided that extend between the liner and screen whereby alternative flow paths in the inner annulus are formed between the screen and the slotted liner. This assembly is successful in preventing bridges from forming; however, the slotted liner requires adequate space between the screen assembly and the wellbore wall, which thereby reduces the production area of the screen assembly.
Thus, there are needs for improved methods and apparatus for completing wells in unconsolidated subterranean zones whereby the migration of formation fines and sand with produced fluids can be economically and permanently prevented while allowing the efficient production of hydrocarbons from the unconsolidated producing zone. In particular, there is a need for a frac/gravel pack apparatus which provides alternative flow paths to prevent voids from forming in the gravel pack and which does not limit or reduce the production area of the screen assembly.
The present invention overcomes the deficiencies of the prior art.
The frac/gravel pack apparatus of the present invention includes a screen assembly having a flow-control assembly disposed therein. A production packer is connected above the screen assembly to support the screen assembly within the wellbore. The screen assembly includes a base member, screens mounted on the base member, and connector subs connecting adjacent base member sections. The connector subs include apertures or ports and shiftable sleeves for closing the ports. The ports are spaced at predetermined intervals along the screen assembly. The shiftable sleeves are in the open position to open the ports during treatment, and the sleeves are shifted to a closed position to close the ports when the flow-control assembly is removed from the well.
The flow-control assembly includes a service assembly and a cross-over or other connection between the service assembly and the work string extending to the surface. The service assembly includes an outer tube, an internal tube, and diverters in the form of caps or shrouds. The outer tube includes externally mounted collet mechanisms and apertures or ports that align with the screen assembly ports. The internal tube is disposed within the outer tube and passes liquid returns to the surface after the returns flow through the screen assembly during gravel packing. The diverters are mounted within the outer tube and cover each port to provide a bridge barrier. Since bridging is most likely to occur at a port, the diverters mounted just inside the outer tube prevent nodes from extending radially across the inner annulus between the service assembly outer tube and internal tube and thereby prevent bridges from forming to block flow through the inner annulus. Therefore, when a bridge builds at one port, the diverter halts the radial formation of the bridge to keep an alternative flow path through the service assembly open to allow the frac fluids or gravel pack slurry to reach lower ports. Externally mounted collet mechanisms on the outer tube are designed to engage and close the shiftable sleeves as the flow-control service assembly is removed from the well after frac treatment and gravel packing are complete.
The present invention features improved methods and apparatus for fracture stimulating and gravel packing wells in unconsolidated subterranean zones, meeting the needs described above and overcoming the deficiencies of the prior art.
The improved methods comprise the steps of placing a screen assembly with a flow-control service assembly disposed therein in an unconsolidated subterranean zone; isolating the outer annulus between the screen assembly and the wellbore wall; and injecting frac fluids or a gravel pack slurry through the service assembly into the outer annulus between the screen assembly and the zone by way of axial ports located at predetermined intervals along the outer tube of the service assembly aligned with ports in the screen assembly.
The unconsolidated formation is fractured during the injection of the frac fluids into the unconsolidated producing zone with proppant being deposited in the fractures. The frac fluid is injected into the formation at a high flow rate through each of the ports, allowing a fairly uniform pressure to be applied at each port location to efficiently and uniformly fracture the zone along the entire interval from top to bottom.
During gravel packing, the particulate material in the slurry is uniformly packed into the outer annulus between the screen assembly and the borehole wall. As bridges form in the outer annulus, the inner annulus, formed between the service assembly outer tube and internal tube, provides alternative flow paths to other ports through which gravel pack slurry can flow to fill any voids formed around the screen assembly, thereby achieving a uniform gravel pack. Diverters covering the service assembly outer tube ports form a radial barrier to prevent the formation of bridges in the inner annulus thereby maintaining the alternative flow paths open through the service assembly so that particulate material can be injected into the outer annulus through lower ports to fill any remaining voids. The permeable pack of particulate material then prevents the migration of formation fines and sand into the wellbore with fluids produced from the unconsolidated zone. Once the frac treatment and gravel packing are complete, the flow-control service assembly is preferably removed from the well. As the flow-control service assembly is raised within the well bore, the outer tube closing mechanisms engage the shiftable sleeves and shift them upward to close the screen assembly ports.
The improved methods and apparatus of the present invention provide more uniform fracture pressures along the entire interval from top to bottom and prevent the formation of voids in the gravel pack, thereby producing an effective fracture and gravel pack. The apparatus of the present invention has the advantage of having a removable flow-control service assembly after frac treatment and gravel packing are complete, and therefore the flow-control service assembly does not limit the available production area within the screen assembly.
It is, therefore, a general object of the present invention to provide improved methods of fracture stimulating and gravel packing wells in unconsolidated subterranean zones. The present invention comprises a combination of features and advantages that enable it to overcome various problems of prior methods and apparatus. The characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.